Shaped articles from orientable polymers and polymer microbeads

ABSTRACT

Disclosed is a shaped article comprising a continuous oriented polymer matrix having dispersed therein microbeads of a polymer which are at least partially boredered by void space, the microbeads being present in an amount of about 5-50% by weight based on the weight of said oriented polymer, said void space occupying about 2-60% by volume of said shaped article. Preferably, the microbeads are cross-linked to an extent they will be resilient and elastic at the orientation temperature, and are coated with a slip agent. The shaped article is preferably in the form of a sheet, fibers, or other molded article, preferably has a relatively low specific gravity and whiteness.

.Iadd.

This application is a reissue application for U.S. Pat. No. 4,994,312,based on U.S. patent application Ser. No. 457,894, filed Dec. 27, 1989,by Larry K. Maier, Elizabeth K. Priebe, Jong S. Lee, Paul E. Woodgate,and Glen C. Smith.

TECHNICAL FIELD

The present invention is directed to shaped articles such as films,sheets, bottles, tubes, fibers and rods having an oriented polymercontinuous phase and polymer microbeads dispersed therein which are atleast partially bordered by voids. The articles have unique propertiesof texture, opaqueness, whiteness in the absence of colorants, andgenerally good physical properties such as thermal stability,durability, and low density.

BACKGROUND OF THE INVENTION

Blends of linear polyesters with other incompatible materials of organicor inorganic nature to form microvoided structures are well-known in theart. U.S. Pat. No. 3,154,461 discloses, for example, linear polyestersblended with, for example, calcium carbonate. U.S. Pat. No. 3,944,699discloses blends of linear polyesters with 3 to 27% of organic materialsuch as ethylene or propylene polymer. U.S. Pat. No. 3,640,944 alsodiscloses the use of poly(ethylene terephthalate) blended with 8%organic material such as polysulfone or poly(4-methyl-1-pentene). U.S.Pat. No. 4,377,616 discloses a blend of polypropylene to serve as thematrix with a small percentage of another and incompatible organicmaterial, nylon, to initiate microvoiding in the polypropylene matrix.U.K. Patent Specification No. 1,563,591 discloses linear polyesterpolymers for making opaque thermoplastic film support in which have beenblended finely divided particles of barium sulfate together with avoid-promoting polyolefin, such as polyethylene, polypropylene orpoly-4-methyl-1-pentene.

The above-mentioned patents show that it is known to use incompatibleblends to form films having paper-like characteristics after such blendshave been extruded into films and the films have been quenched,biaxially oriented and beat set. The minor component of the blend, dueto its incompatibility with the major component of the blend, upon meltextrusion into film forms generally spherical particles each of whichinitiates a microvoid in the resulting matrix formed by the majorcomponent. The melting points of the void initiating particles, in theuse of organic materials, should be above the glass transitiontemperature of the major component of the blend and particularly at thetemperature of biaxial orientation.

As indicated in U.S. Pat. No. 4,377,616, spherical particles initiatevoids of unusual regularity and orientation in a stratified relationshipthroughout the matrix material after biaxial orientation of the extrudedfilm. Each void tends to be of like shape, not necessarily of like sizesince the size depends upon the size of the particle.

Ideally, each void assumes a shape defined by two opposed and edgecontacting concave disks. In other words, the voids tend to have alens-like or biconvex shape. The voids are oriented so that the twomajor dimensions are aligned in correspondence with the direction oforientation of the film structure. One major dimension is aligned withmachine direction orientation, a second major dimension is aligned withthe transverse direction orientation, and a minor dimensionapproximately corresponds to the cross-section dimension of thevoid-initiating particle.

The voids generally tend to be closed cells, and thus there is virtuallyno path open from one side of a biaxially oriented film to the otherside through which liquid or gas can traverse. The term "void" is usedherein to mean devoid of solid matter, although it is likely the "voids"contain a gas.

Upon biaxial orientation of the resulting extruded film, the filmbecomes white and opaque, the opacity resulting from light beingscattered from the walls of the microvoids. The transmission of lightthrough the film becomes lessened with increased number and withincreased size of the microvoids relative to the size of a particlewithin each microvoid.

Also, upon biaxial orientation, a matte finish on the surface of thefilm results, as discussed in U.S. Pat. No. 3,154,461. The particlesadjacent the surfaces of the film tend to be incompressible and thusform projections without rupturing the surface. Such matte finishesenable the film to be written upon with pencil or with inks, crayons,and the like.

U.S. Pat. No. 3,944,699 also indicates that the extrusion, quenching andstretching of the film may be effected by any process which is known inthe art for producing oriented film, such as by a flat film process or abubble or tubular process. The flat film process involves extruding theblend through a slit dye and rapidly quenching the extruded web upon achilled casting drum so that the polyester component of the film isquenched into the amorphous state, The quenched film is then biaxiallyoriented by stretching in mutually perpendicular directions at atemperature above the glass transition temperature of the polyester. Thefilm may be stretched in one direction and then in a second direction ormay be simultaneously stretched in both directions. After the film hasbeen stretched it is heat set by heating to a temperature sufficient tocrystallize the polyester while restraining the film against retractionin both directions of stretching.

Paper is essentially a non-woven sheet of more or less randomly arrayedfibers. The key properties of these structures are opacity, texture,strength, and stability. Natural polymers are generally weaker and lessstable. A serious problem, for example, is brightness reversion orfading of papers and fibers.

Although there are many ways to produce opaque media, this invention isconcerned with creating opacity by stretching or orienting plasticmaterials to induce microvoids which scatter light, preferably white andultraviolet light. A large body of prior art deals with this technique,wherein a plurality of inorganic solid particles are used as thedispersed phase, around which the microvoids form. Some significantproblems associated with this approach are: (1) agglomeration andparticle size control, (2) abrasive wear of extrusion equipment, guides,and cutters, (3) high specific gravity of these solids, (4) poor voidnucleation around the solid particles due to the low thermal contractionof solids relative to liquids and polymer wetting and adhesion to thesolid surfaces, (5) cost of these materials on a volume basis, and (6)handling and processing problems in general.

Of particular interest is U.S. Pat. No. 4,770,931 which is directed toarticles comprising a continuous polyester phase having dispersedtherein microbeads of cellulose acetate which are at least partiallybordered by void space. The present invention is unexpected, however, inthat while the geometries are similar, the cross-linked beads disclosedherein cavitate more efficiently generating higher void fractions andimproved properties per weight of added beads. Also, the compositions ofthis invention have superior thermal and chemical stability, whencompared with the prior art, especially the cellulose esters. Also, ofparticular interest is U.S. Pat. No. 4,320,207 which discloses orientedpolyester film containing pulverized cross-linked polymers

DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a perspective view in section illustrating an embodiment ofthe present invention;

FIG. 2 is a perspective view in section illustrating another embodimentof the present invention;

FIG. 3 is a perspective view illustrating still another embodiment ofthe present invention;

FIG. 4 is a section of a shaped article in the form of a bottle;

FIG. 5 is an enlarged section view illustrating a crosslinked polymermicrobead entrapped in a void in a polyester continuous matrix;

FIG. 6 is a sectional view taken along lines 6--6 of FIG. 5;

FIG. 7 is a sectional view similar to FIG. 5 illustrating a modificationof the present invention; and

FIG. 8 is a graphical representation illustrating how the size ofmicrovoids surrounding microbeads changes with respect to stretch ratio.

DESCRIPTION OF THE INVENTION

In accordance with the present invention, shaped articles are providedwhich have unique properties such as texture, opacity, low density,whiteness, etc. The articles are especially useful when in the form offilm or sheet material (e.g., as a paper substitute) or when in the formof a biaxially oriented bottle (beverage container).

An important aspect of this invention is that during melt processing theorientable polymer does not react chemically or physically with themicrobead polymer and/or its coating in such a way as to cause one ormore of the following to occur to a significant or unacceptable degree:(a) alteration of the crystallization kinetics of the matrix polymermaking it difficult to orient, (b) destruction of the matrix polymer,(c) destruction of the microbeads, (d) adhesion of the microbeads to thematrix polymer, or (e) generation of undesirable reaction products, suchas toxic or high-color moieties.

Referring to the drawings, FIG. 1 illustrates a shaped article in theform of a sheet 10 which has been biaxially oriented [biaxiallystretched, i.e., stretched in both the longitudinal (X) and transverse(Y) directions], as indicated by the arrows. The sheet 10 is illustratedin section, showing microbeads of polymer 12 contained within circularvoids 14 in the polymer continuous matrix 16. The voids 14 surroundingthe microbeads 12 are theoretically doughnut-shaped, but are often ofirregular shape. Often, a line drawn perpendicular to and through thearticle will penetrate several voids and possibly some microbeads.

FIG. 2 also illustrates a shaped article in the form of a sheet 20 whichhas been unidirectionally oriented (stretched in one direction only, asindicated by the arrow). Microbeads of polymer 22 are contained betweenmicrovoids 24 and 24'. The microvoids in this instance form at oppositesides of the microbeads as the sheet is stretched. Thus, if thestretching is done in the machine direction (X) as indicated by thearrow, the voids will form on the leading and trailing sides of themicrobeads. This is because of the unidirectional orientation as opposedto the bidirectional orientation of the sheet shown in FIG. 1. This isthe only difference between FIG. 1 and 2. Note particularly the bumpytexture of the surfaces.

FIG. 3 illustrates a shaped article in the form of a fiber or rod 30which has been oriented by stretching in the lengthwise (X) direction.The microbeads 32 of cross-linked polymer are bordered by microvoids 34and 34'.

FIG. 4 illustrates a section of the wall of a shaped article 40 such asa bottle or wire coating. Due to the bidirectional orientation orstretching, the microvoids 42 are generally doughnut-shaped, surroundingthe microbeads 44, in a manner similar to that. shown in FIG. 1.

FIGS. 5 and 6 are sectional views illustrating enlargement of a sectionof a shaped article according to this invention, microbead 50 beingentrapped within polymer continuous matrix 52 and encircled by void 54.These structures result from the shaped article being stretched in the Xand Y directions.

FIG. 7 is a view similar to FIG. 5, except illustrating in enlarged formmicrobead 60 entrapped in polymer continuous matrix 62, having formed onopposite sides thereof microvoids 64 and 64', which are formed as theshaped article is stretched in the direction of the arrow X.

FIG. 8 is an enlargement illustrating the manner in which microvoids areformed in the polymer continuous matrix as the shaped article isstretched or oriented. The formation of the microvoids 70 and 70' aroundmicrobeads 72 is illustrated on a stretch ratio scale as the shapedarticle is stretched up to 4 times its original dimension. For example,as the article is stretched 4 times its original dimension in the Xdirection (4X), the voids 70 and 70' would extend to the points 74 and74' respectively.

There are two aspects of the present invention. First, the microbeadsare preferably cross-linked to given them resiliency and elasticity.Second, the microbeads are coated with a "slip agent" to permit easiersliding with respect to the matrix polymer to thereby result in moremicrovoiding. Although both aspects are believed to be unique and resultin improved results to an extent, it is preferred that the microbeads beboth cross-linked and coated with a slip agent. Thus, the microbeadswill be often described herein as being both cross-linked and coatedwith a slip agent.

The present invention provides shaped articles comprising a continuousthermoplastic polymer phase having dispersed therein microbeads ofpolymer which are at least partially bordered by voids, the microbeadsof polymer having a size of about 0.1-50 microns, preferably about. 2-20microns, and being present in an amount of about 5-50% by weight basedon the weight of continuous phase polymer, the voids occupying about2-60% by volume of the shaped article. The matrix polymer containing thegenerally spherical polymer microbeads which, according to one aspect ofthe invention, are cross-linked to the extent of having a resiliency orelasticity at orientation temperatures of the matrix polymer such that agenerally spherical shape of the cross-linked polymer is maintainedafter orientation of the matrix polymer. The composition of the shapedarticle when consisting only of the polymer continuous phase andmicrobeads bordered by voids, is characterized by having a specificgravity of less than 1.20, preferably about 0.3-1.0, preferably aKubelka-Munk R value (infinite thickness) of about 0.90 to about 1.0,and preferably the following Kubelka-Munk values when formed into a 3mil thick film:

    ______________________________________                                        Opacity          about 0.75 to about 1.0                                      SX               25 or less                                                   KX               about 0.001 to 0.2                                           Ti               about 0.02 to 1.0                                            ______________________________________                                    

wherein the opacity values indicate that the article is opaque, the SXvalues indicate a large amount of light scattering through the thicknessof the article, the KX values indicate a low amount of light absorptionthrough the thickness of the article, and the Ti values indicate a lowlevel amount of internal transmittance of the thickness of the article.The R (infinite thickness) values indicate a large amount of lightreflectance.

Obviously, the Kubelka-Munk values which are dependent on thickness ofthe article must be specified at a certain thickness. Although theshaped articles themselves may be very thin, e.g., less than 1 mil orthey may be thicker, e.g., 20 mils, the Kubelka-Munk values, except forR infinity, are specified at 3 mils and in the absence of any additiveswhich would effect optical properties. Thus, to determine whether shapedarticles have the optical properties called for, the polyestercontaining microbeads at least partially bordered by voids, withoutadditives, should be formed in a 3 mils thick film for determination ofKubelka-Munk values.

The shaped articles according to this invention are useful, for example,when in the forms of sheets or films, bottles, ribbons, fibers or rods,wire coatings, etc. In the absence of additives or colorants, they arevery white, have a very pleasant feel or hand and receptive to inkespecially the polyester matrices from writing instruments, especiallyconventional ball point pens. In fact, one of the most important usescontemplated for the present invention is as a synthetic paper forwriting on or for prints such as drawings and other graphic,photographic and printing applications. The shaped articles are veryresistant to wear, moisture, oil, tearing, etc.

The shaped article is preferably in the form of a paper-like sheethaving a thickness of about 0.10-20 mils in thickness. The shapedarticle may also be an oriented bottle made by injection blow molding,or may be in the form of a fiber or rod. Preferably, the article is madeby biaxial orientation using procedures well known in the art.

The products made in accordance with this invention are very durable.For example, when made into biaxially oriented films, the resultantsynthetic papers are strong, ultra-white, highly-opaque, andlong-lasting. Such papers are suitable for "archival" records and willretain their properties for very long periods of time, even whencompared to the so-called "archival quality" papers of today. Not onlyare the synthetic papers of this invention extremely white, they arevirtually free of the problem which plaques cellulose-based papers,i.e., "brightness reversion" or yellowing with time.

The products of this invention are environmentally desirable products.They are long lasting, durable, and recyclable. They can be made from"recycled" materials; e.g., poly(ethylene terephthalate) beveragebottles. Also, upon incineration, less than 1% ash and no undesirablessuch as chlorine, cyanides, etc. are found. Finally, if they are putinto landfills, they will not make toxic gases or liquids, and they willnot threaten the quality of our air or ground water.

The continuous phase polymer may be any article forming polymer such asa polyester capable of being cast into a film or sheet, spun intofibers, extruded into rods or extrusion, blow-molded into containerssuch as bottles, etc. The polyesters should have a glass transitiontemperature between about 50° C. and about 150°C., preferably about60°-100° C., should be orientable, and have an I.V. of at least 0.50,preferably 0.6 to 0.9. Suitable polyesters include those produced fromaromatic, aliphatic or cycloaliphatic dicarboxylic acids of 4-20 carbonatoms and aliphatic or alicyclic glycols having from 2-24 carbon atoms.Examples of suitable dicarboxylic acids include terephthalic,isophthalic, phthalic, naphthalene dicarboxylic acid, succinic,glutaric, adipic, azelaic, sebacic, fumaric, maleic, itaconic,1,4-cyclohexanedicarboxylic, sodiosulfoisophthalic and mixtures thereof.Examples of suitable glycols include ethylene glycol, propylene glycol,butanediol, pentanediol, hexanediol, 1,4-cyclohexanedimethanol,diethylene glycol, other polyethylene glycols and mixtures thereof. Suchpolyesters are well known in the art and may be produced by well-knowntechniques, e.g., those described in U.S. Pat. Nos. 2,465,319 and2,901,466. Preferred continuous matrix polymers are those having repeatunits from terephthalic acid or naphthalene dicarboxylic acid and atleast one glycol selected from ethylene glycol, 1,4-butanediol and1,4-cyclohexanedimethanol. Poly(ethylene terephthalate), which may bemodified by small amounts of other monomers, is especially preferred.Polypropylene is also useful. Other suitable polyesters include liquidcrystal copolyesters formed by the inclusion of a suitable amount of aco-acid component such as stilbene dicarboxylic acid. Examples of suchliquid crystal copolyesters are those disclosed in U.S. Pat. Nos.4,420,607, 4,459,402 and 4,468,510.

Suitable cross-linked polymers for the microbeads are polymerizableorganic materials which are members selected from the group consistingof an alkenyl aromatic compound having the general formula ##STR1##wherein Ar represents an aromatic hydrocarbon radical, or an aromatichalohydrocarbon radical of the benzene series and R is hydrogen or themethyl radical; acrylate-type monomers include monomers of the formula##STR2## wherein R is selected from the group consisting of hydrogen andan alkyl radical containing from about 1 to 12 carbon atoms and R' is oselected from the group consisting of hydrogen and methyl; copolymers ofvinyl chloride and vinylidene chloride, acrylonitrile and vinylchloride, vinyl bromide, vinyl esters having the formula ##STR3##wherein R is an alkyl radical containing from 2 to 18 carbon atoms;acrylic acid, methacrylic acid, itaconic acid, citraconic acid, maleicacid, fumaric acid, oleic acid, vinylbenzoic acid; the syntheticpolyester resins which are prepared by reacting terephthalic acid anddialkyl terephthalics or ester-forming derivatives thereof, with aglycol of the series HO(CH₂)_(n) OH, wherein n is a whole number withinthe range of 2-10 and having reactive olefinic linkages within thepolymer molecule, the hereinabove described polyesters which includecopolymerized therein up to 20 percent by weight of a second acid orester thereof having reactive olefinic unsaturation and mixturesthereof, and a cross-linking agent selected from the group consisting ofdivinylbenzene, diethylene glycol dimethacrylate, oiallyl fumarate,diallyl phthalate and mixtures thereof.

Examples of typical monomers for making the cross-linked polymer includestyrene, butyl acrylate, acrylamide, acrylonitrile, methyl methacrylate,ethylene glycol dimethacrylate, vinyl pyridine, vinyl acetate, methylacrylate, vinylbenzyl chloride, vinylidene chloride, acrylic acid,divinylbenzene, acrylamidomethylpropane sulfonic acid, vinyl toluene,etc. Preferably, the cross-linked polymer is polystyrene or poly(methylmethacrylate). Most preferably, it is polystyrene and the cross-linkingagent is divinylbenzene. Processes well known in the art yieldnon-uniformly sized particles, characterized by broad particle sizedistributions. The resulting beads can be classified by screening toproduce beads spanning the range of the original distribution of sizes.Other processes such as suspension polymerization, limited coalescence,directly yield very uniformly sized particles.

Suitable slip agents or lubricants include colloidal silica, colloidalalumina, and metal oxides such as tin oxide and aluminum oxide. Thepreferred slip agents are colloidal silica and alumina, most preferably,silica. The cross-linked polymer having a coating of slip agent may beprepared by procedures well known in the art. For example, conventionalsuspension polymerization processes wherein the slip agent is added tothe suspension is preferred. As the slip agent, colloidal silica ispreferred.

It is preferred to use the "limited coalescence" technique for producingthe coated, cross-linked polymer microbeads. This process is describedin detail in U.S. Pat. No. 3,615,972, incorporated herein by reference.Preparation of the coated microbeads for use in the present inventiondoes not utilize a blowing agent as described in this patent, however.

The following general procedure may be utilized in a limited coalescencetechnique.

1. The polymerizable liquid is dispersed within an aqueous nonsolventliquid medium to form a dispersion of droplets having sizes not largerthan the size desired for the polymer globules, whereupon

2. The dispersion is allowed to rest and to reside with only mild or noagitation for a time during which a limited coalescence of the disperseddroplets takes place with the formation of a lesser number of largerdroplets, such coalescence being limited due to the composition of thesuspending medium, the size of the dispersed droplets thereby becomingremarkably uniform and of a desired magnitude, and

3. The uniform droplet dispersion is then stabilized by addition ofthickening agents to the aqueous suspending medium, whereby theuniform-sized dispersed droplets are further protected againstcoalescence and are also retarded from concentrating in the dispersiondue to difference in density of the disperse phase and continuous phase,and

4. The polymerizable liquid or oil phase in such stabilized dispersionis subjected to polymerization conditions and polymerized, wherebyglobules of polymer are obtained having spheroidal shape and remarkablyuniform and desired size, which size is predetermined principally by thecomposition of the initial aqueous liquid suspending medium.

The diameter of the droplets of polymerizable liquid, and hence thediameter of the beads of polymer, can be varied predictably, bydeliberate variation of the composition of the aqueous liquiddispersion, within the range of from about one-half of a micron or lessto about 0.5 centimeter. For any specific operation, the range ofdiameters of the droplets of liquid,. and hence of polymer beads, has afactor in the order of three or less as contrasted to factors of 10 ormore for diameters of droplets and beads prepared by usual suspensionpolymerization methods employing critical agitation procedures. Sincethe bead size,. e.g., diameter, in the present method is determinedprincipally by the composition of the aqueous dispersion, the mechanicalconditions, such as the degree of agitation, the size and design of theapparatus used, and the scale of operation, are not highly critical.Furthermore, by employing the same composition, the operations can berepeated, or the scale of operations can be changed, and substantiallythe same results can be obtained.

The present method is carried out by dispersing one part by volume of apolymerizable liquid into at least 0.5, preferably from 0.5 to about 10or more, parts by volume of a nonsolvent aqueous medium comprising waterand at least the first of the following ingredients.

1. A water-dispersible, water-insoluble solid colloid, the particles ofwhich, in aqueous, dispersion, have dimensions in the order of fromabout 0.008 to about 50 microns, which particles tend to gather at theliquid-liquid interface or are caused to do so by the presence of

2. A water-soluble "promotor" that affects the "hydrophilic-hydrophobicbalance" of the solid colloid particles; and/or

3. An electrolyte; and/or

4. Colloid-active modifiers such as peptizing agents, surface-activeagents and the like; and, usually,

5. A water-soluble, monomer-insoluble inhibitor of polymerization.

The water-dispersible, water-insoluble solid colloids can be inorganicmaterials such as metal salts or hydroxides or clays, or can be organicmaterials such as raw starches, sulfonated cross-linked organic highpolymers, resinous polymers and the like.

The solid colloidal material must be insoluble but dispersible in waterand both insoluble and nondispersible in, but wettable by, thepolymerizable liquid. The solid colloids must be much more hydrophilicthan oleophilic so as to remain dispersed wholly within the aqueousliquid. The solid colloids employed for limited coalescence are oneshaving particles that, in the aqueous liquid, retain a relatively rigidand discrete shape and size within the limits stated. The particles maybe greatly swollen and extensively hydrated, provided that the swollenparticle retains a definite shape, in which case the effective size isapproximately that of the swollen particle. The particles can beessentially single molecules, as in the case of extremely high molecularweight cross-linked resins, or can be aggregates of many molecules.Materials that disperse in water to form true or colloidal solutions inwhich the particles have a size below the range stated or in which theparticles are so diffuse as to lack a discernible shape and dimensionare not suitable as stabilizers for limited coalescence. The amount ofsolid colloid that is employed is usually such as corresponds to fromabout 0.01 to about 10 or more grams per 100 cubic centimeters of thepolymerizable liquid.

In order to function as a stabilizer for the limited coalescence of thepolymerizable liquid droplets, it is essential that the solid colloidmust tend to collect with the aqueous liquid at the liquidliquidinterface, i.e., on the surface of the oil droplets. (The term "oil" isoccasionally used herein as generic to liquids that are insoluble inwater.) In many instances, it is desirable to add a "promoter" materialto the aqueous composition to drive the particles of the solid colloidto the liquid-liquid interface. This phenomenon is well known in theemulsion art, and is here applied to solid colloidal particles, as aexpanded of adjusting the "hydrophilichydrophobic balance."

Usually, the promoters are organic materials that have an affinity forthe solid colloid and also for the oil droplets and that are capable ofmaking the solid colloid more oleophilic. The affinity for the oilsurface is usually due to some organic portion of the promoter moleculewhile affinity for the solid colloid is usually due to oppositeelectrical charges. For example, positively charged complex metal saltsor hydroxides, such as aluminum hydroxide, can be promoted by thepresence of negatively charged organic promoters such as water-solublesulfonated polystyrenes, alignates and carboxymethyleellulose.Negatively charged colloids, such as Bentonite, are promoted bypositively charged promoters such as tetramethyl ammonium hydroxide orchloride or water-soluble complex resinous amine condensation productssuch as the water-soluble condensation products of diethanolamine andadipic acid, the water-soluble condensation products of ethylene oxide,urea and formaldehyde, and polyethylenimine. Amphoteric materials suchas proteinaceous materials like gelatin, glue, casein, albumin, glutinand the like, are effective promoters for a wide variety of colloidalsolids. Nonionic materials like methoxycellulose are also effective insome instances. Usually, the promoter need be used only to the extent ofa few parts per million of aqueous medium although larger proportionscan often be tolerated. In some instances, ionic materials normallyclassed as emulsifiers, such as soaps, long chain sulfates andsulfonates and the long chain quaternary ammonium compounds, can also beused as promoters for the solid colloids, but care must be taken toavoid causing the formation of stable colloidal emulsions of thepolymerizable liquid and the aqueous liquid medium.

An effect similar to that of organic promoters is often obtained withsmall amounts of electrolytes, e.g., water-soluble, ionizable alkalies,acids and salts, particularly those having polyvalent ions. These areespecially useful when the excessive hydrophilic or insufficientoleophilic characteristic of the colloid is attributable to excessivehydration of the colloid structure. For example, a suitably cross-linkedsulfonated polymer of styrene is tremendously swollen and hydrated inwater. Although the molecular structure contains benzene rings whichshould confer on the colloid some affinity for the oil phase in thedispersion, the great degree of hydration causes the colloidal particlesto be enveloped in a cloud of associated water. The addition of asoluble, ionizable polyvalent cationic compound, such as an aluminum orcalcium salt, to the aqueous composition causes extensive shrinking ofthe swollen colloid with exudation of a part of the associated water andexposure of the organic portion of the colloid particle, thereby makingthe colloid more oleophilic.

The solid colloidal particles whose hydrophilichy-drophobic balance issuch that the particles tend to gather in the aqueous phase at theoil-water interface, gather on the surface of the oil droplets andfunction as protective agents in the phenomenon of limited coalescence.

Other agents that can be employed in an already known manner to effectmodification of the colloidal properties of the aqueous composition arethose materials known in the art as peptizing agents, flocculating anddeflocculating agents, sensitizers, surface active agents and the like.

It is sometimes desirable to add to the aqueous liquid a few parts permillion of a water-soluble, oil-insoluble inhibitor of polymerizationeffective to prevent the polymerization of monomer molecules that mightdiffuse into the aqueous liquid or that might be absorbed by colloidmicelles and that, if allowed to polymerize in the aqueous phase, wouldtend to make emulsion-type polymer dispersions instead of, or inaddition to, the desired bead or pearl polymers.

The aqueous medium containing the water-dispersible solid colloid isthen admixed with the liquid polymerizable material in such a way as todisperse the liquid polymerizable material as small droplets within theaqueous medium. This dispersion can be accomplished by any usual means,e.g., by mechanical stirrers or shakers, by pumping through jets, byimpingement, or by other procedures causing subdivision of thepolymerizable material into droplets in a continuous aqueous medium.

The degree of dispersion, e.g., by agitation is not critical except thatthe size of the dispersed liquid droplets must be no larger, and ispreferably much smaller, than the stable droplet size expected anddesired in the stable dispersion. When such condition has been attained,the resulting dispersion is allowed to rest with only mild, gentlemovement, if any, and preferably without agitation. Under such quiescentconditions, the dispersed liquid phase undergoes a limited degree ofcoalescence.

"Limited coalescence" is a phenomenon wherein droplets of liquiddispersed in certain aqueous suspending media coalesce, with formationof a lesser number of larger droplets, until the growing droplets reacha certain critical and limiting size, whereupon coalescencesubstantially ceases. The resulting droplets of dispersed liquid, whichcan be as large as 0.3 and sometimes 0.5 centimeter in diameter, arequite stable as regards further coalescence and are remarkably uniformin size. If such a large droplet dispersion be vigorously agitated, thedroplets are fragmented into smaller droplets. The fragmented droplets,upon quiescent standing, again coalesce to the same limited degree andform the same uniform-sized, large droplet, stable dispersion. Thus, adispersion resulting from the limited coalescence comprises droplets ofsubstantially uniform diameter that are stable in respect to furthercoalescence.

The principles underlying this phenomenon have now been adapted to causethe occurrence of limited coalescence in a deliberate and predictablemanner in the preparation of dispersions of polymerizable liquids in theform of droplets of uniform and desired size.

In the phenomenon of limited coalescence the small particles of solidcolloid tend to collect with the aqueous liquid at the liquid-liquidinterface, i.e., on the surface of the oil droplets. It is thought thatdroplets which are substantially covered by such solid colloid arestable to coalescence while droplets which are not so covered are notstable. In a given dispersion of a polymerizable liquid the totalsurface area of the droplets is a function of the total volume of theliquid and the diameter of the droplets. Similarly, the total surfacearea barely coverable by the solid colloid, e.g., in a layer oneparticle thick, is a function of the amount of the colloid and thedimensions of the particles thereof. In the dispersion as initiallyprepared, e.g., by agitation, the total surface area of thepolymerizable liquid droplets is greater than can be covered, by thesolid colloid. Under quiescent conditions, the unstable droplets beginto coalesce. The coalescence results in a decrease in the number of oildroplets and a decrease in the total surface area thereof up to a pointat which the amount of colloidal solid is barely sufficientsubstantially to cover the total surface of the oil droplets, whereuponcoalescence substantially ceases.

If the solid colloidal particles do not have nearly identicaldimensions, the average effective dimension can be estimated bystatistical methods. For example, the average effective diameter ofspherical particles can be computed as the square root of the average ofthe squares of the actual diameters of the particles in a representativesample.

It is usually beneficial to treat the uniform droplet suspensionprepared as described above to render the suspension stable againstcongregation of the oil droplets.

This further stabilization is accomplished by gently admixing with theuniform droplet dispersion an agent capable of greatly increasing theviscosity of the aqueous liquid. For this purpose, there may be used anywater-soluble or water-dispersible thickening agent that is insoluble inthe oil droplets and that does not remove the layer of solid colloidalparticles covering the surface of the oil droplets at the oil-waterinterface. Examples of suitable thickening agents are sulfonatedpolystyrene (water-dispersible, thickening grade), hydrophilic clayssuch as Bentonite, digested starch, natural gums, carboxy-substitutedcellulose ethers and the like. Often the thickening agent is selectedand employed in such quantities as to form a thixotropic gel in whichare suspended the uniform-sized droplets of the oil. In other words, thethickened liquid generally should be non-Newtonian in its fluidbehavior, i.e., of such a nature as to prevent rapid movement of thedispersed droplets within the aqueous liquid by the action ofgravitational force due to the difference in density of the phases. Thestress exerted on the surrounding medium by a suspended droplet is notsufficient to cause rapid movement of the droplet within suchnon-Newtonian media. Usually, the thickener agents are employed in suchproportions relative to the aqueous liquid that the apparent viscosityof the thickened aqueous liquid is in the order of at least 500centipoises (usually determined by means of a Brookfield viscosimeterusing the No. 2 spindle at 30 r.p.m.). The thickening agent ispreferably prepared as a separate concentrated aqueous composition thatis then carefully blended with the oil droplet dispersion.

The resulting thickened dispersion is capable of being handled, e.g.,passed through pipes, and can be subjected to polymerization conditionssubstantially without mechanical change in the size or shape of thedispersed oil droplets.

The resulting dispersions are particularly well suited for use incontinuous polymerization procedures that, can be carried out in coils,tubes and elongated vessels adapted for continuously introducing thethickened dispersions into one end and for continuously withdrawing themass of polymer beads from the other end. The polymerization step isalso practiced in batch manner.

The order of the addition of the constituents to the polymerizationusually is not critical, but beneficially it is more convenient to addto a vessel the water, dispersing agent, and incorporated theoil-soluble catalyst to the monomer mixture, and subsequently add withagitation the monomer phase to the water phase.

The following is an example illustrating a procedure for preparing thecross-linked polymeric microbeads coated with slip agent. In thisexample, the polymer is polystyrene cross-linked with divinylbenzene.The microbeads have a coating of silica. The microbeads are prepared bya procedure in which monomer droplets containing an initiator are sizedand heated to give solid polymer spheres of the same size as the monomerdroplets. A water phase is prepared by combining 7' liters of distilledwater, 1.5 g potassium dichromate (polymerization inhibitor for theaqueous phase), 250 g polymethylaminoethanol adipate (promoter), and 350g LUDOX (a colloidal suspension containing 50% silica sold by DuPont. Amonomer phase is prepared by combining 3317 g styrene, 1421 gdivinylbenzene (55% active crosslinking agent; other 45% is ethyl vinylbenzene which forms part of the styrene polymer chain) and 45 g VAZO 52(a monomer-soluble initiator sold by DuPont). The mixture is passedthrough a homogenizer to obtain 5 micron droplets. The suspension isheated overnight at 52° C. to give 4.3 kg of generally sphericalmicrobeads having an average diameter of about 5 microns with narrowsize distribution (about 2-10 microns size distribution) The molproportion of styrene and ethyl vinyl benzene to divinylbenzene is about6.1%. The concentration of divinylbenzene can be adjusted up or down toresult in about 2.5-50% (preferably 10-40%) crosslinking by the activecross-linker. Of course, monomers other than styrene and divinylbenzenecan be used in similar suspension polymerization processes known in theart. Also, other initiators and promoters may be used as known in theart. Also, slip agents other than silica may also be used. For example,a number of LUDOX colloidal silicas are available from DuPont. LEPANDINcolloidal alumina is available from Degussa. NALCOAG colloidal silicasare available from Nalco and tin oxide and titanium oxide are alsoavailable from Nalco.

Normally, for the polymer to have suitable physical properties such asresiliency, the polymer is crosslinked. In the case of styrenecrosslinked with divinylbenzene, the polymer is about 2.5-50%cross-linked, preferably about 20-40% cross-linked. By percentcross-linked, it is meant the mol % of crosslinking agent based on theamount of primary monomer. Such limited crosslinking produces microbeadswhich are sufficiently coherent to remain intact during orientation ofthe continuous polymer. Beads of such crosslinking are also resilient,so that when they are deformed (flattened) during orientation bypressure from the matrix polymer on opposite sides of the microbeads,they subsequently resume their normal spherical shape to produce thelargest possible voids around the microbeads to thereby produce articleswith less density.

The microbeads are referred to herein as having a coating of a "slipagent". By this term it is meant that the friction at the surface of themicrobeads is greatly reduced. Actually, it is believed this is causedby the silica acting as miniature ball bearings at the surface. Slipagent may be formed on the surface of the microbeads during theirformation by including it in the suspension polymerization mix.

Microbead size is regulated by the ratio of silica to monomer. Forexample, the following ratios produce the indicated size microbead:

    ______________________________________                                                                  Slip Agent                                          Microbead Size,                                                                              Monomer,   (Silica)                                            Microns        Parts by Wt.                                                                             Parts by Wt.                                        ______________________________________                                        2              10.4       1                                                   5              27.0       1                                                   20             42.4       1                                                   ______________________________________                                    

The microbeads of cross-linked polymer range in size from about 0.1-50microns, and are present in an amount of about 5-50% by weight based onthe weight of the polyester. Microbeads of polystyrene should have a Tgof at least 20° C. higher than the Tg of the continuous matrix polymerand are hard compared to the continuous matrix polymer.

Elasticity and resiliency of the microbeads generally results inincreased voiding, and it is preferred to have the Tg of the microbeadsas high above that of the matrix polymer as possible to avoiddeformation during orientation. It is not believed that there is apractical advantage to cross-linking above the point of resiliency andelasticity of the microbeads.

The microbeads of cross-linked polymer are at least partially borderedby voids. The void space in the shaped article should occupy about2-60%, preferably about 30-50%, by volume of the shaped article.Depending on the manner in which the shaped articles are made, the voidsmay completely encircle the microbeads, e.g., a void may be in the shapeof a doughnut (or flattened doughnut) encircling a microbead, or thevoids may only partially border the microbeads, e.g., a pair of voidsmay border a microbead on opposite sides.

The invention does not require but permits the use or addition of aplurality of organic and inorganic materials such as fillers, pigments,antiblocks, anti-stats, plasticizers, dyes, stabilizers, nucleatingagents, optical brighteners, etc. These materials may be incorporatedinto the matrix phases, into the dispersed phases, or may exist asseparate dispersed phases.

During stretching the voids assume characteristic shapes from thebalanced biaxial orientation of paper-like films to the uniaxialorientation of microvoided/satin-like fibers. Balanced microvoids arelargely circular in the plane of orientation while fiber microvoids areelongated in the direction of the fiber axis. The size of the microvoidsand the ultimate physical properties depend upon the degree and balanceof the orientation, temperature and rate of stretching, crystallizationkinetics, the size distribution of the microbeads, and the like.

The shaped articles according to this invention are prepared by

(a) forming a mixture of molten continuous matrix polymer andcross-linked polymer wherein the cross-linked polymer is a multiplicityof microbeads uniformly dispersed throughout the matrix polymer, thematrix polymer being as described hereinbefore, the cross-linked polymermicrobeads being as described hereinbefore,

(b) forming a shaped article from the mixture by extrusion, casting ormolding,

(c) orienting the article by stretching to form microbeads ofcross-linked polymer uniformly distributed throughout the article andvoids at least partially bordering the microbeads on sides thereof inthe direction, or directions of orientation.

The mixture may be formed by forming a melt of the matrix polymer andmixing therein the cross-linked polymer. The cross-linked polymer may bein the form of solid or 22 semi-solid microbeads. Due to theincompatibility between the matrix polymer and cross-linked polymer,there is no attraction or adhesion between them, and they becomeuniformly dispersed in the matrix polymer upon mixing.

When the microbeads have become uniformly dispersed in the matrixpolymer, a shaped article is formed by processes such as extrusion,casting or molding. Examples of extrusion or casting would be extrudingor casting a film or sheet, and an example of molding would be injectionor reheat blow-molding a bottle. Such forming methods are well known inthe art. If sheets or film material are cast or extruded, it isimportant that such article be oriented by stretching, at least in onedirection. Methods of unilaterally or bilaterally orienting sheet orfilm material are well known in the art. Basically, such methodscomprise stretching the sheet or film at least in the machine orlongitudinal direction after it is cast or extruded an amount of about1.5-10 times its original dimension. Such sheet or film may also bestretched in the transverse or cross-machine direction by apparatus andmethods well known in the art, in amounts of generally 1.5-10 (usually3-4 for polyesters and 6-10 for polypropylene) times the originaldimension. Such apparatus and methods are well known in the art and aredescribed in such U.S. Pat. No. 3,903,234, incorporated herein byreference.

If the shaped article is in the form of a bottle, orientation isgenerally biaxial as the bottle is stretched in all directions as it isblow-molded. Such formation of bottles is also well known in the art.See, for example, U.S. Pat. No. 3,849,530, incorporated herein byreference.

The voids, or void spaces, referred to herein surrounding the microbeadsare formed as the continuous matrix polymer is stretched at atemperature above the Tg of the matrix polymer. The microbeads ofcrosslinked polymer are relatively hard compared to the continuousmatrix polymer. Also, due to the incompatibility and immiscibilitybetween the microbead and the matrix polymer, the continuous matrixpolymer slides over the microbeads as it is stretched, causing voids tobe formed at the sides in the direction or directions of stretch, whichvoids elongate as the matrix polymer continues to be stretched. Thus,the final size and shape of the voids depends on the direction(s) andamount of stretching. If stretching is only in one direction, microvoidswill form at the sides of the microbeads in the direction of stretching.If stretching is in two directions (bidirectional stretching), in effectsuch stretching has vector components extending radially from any givenposition to result in a doughnut-shaped void surrounding each microbead.

The preferred preform stretching operation simultaneously opens themicrovoids and orients the matrix material. The final product propertiesdepend on and can be controlled by stretching time-temperaturerelationships, and on the type and degree of stretch. For maximumopacity and texture, the stretching is done just above the glasstransition temperature of the matrix polymer. When stretching is done inthe neighborhood of the higher glass transition temperature, both phasesmay stretch together and opacity decreases In the former case, thematerials are pulled apart, a mechanical anticompatibilization process.Two examples are high-speed melt spinning of fibers and melt blowing offibers and films to form non-woven/spun-bonded products. In summary, thescope of this invention includes the complete range of formingoperations just described.

In general, void formation occurs independent of, and does not require,crystalline orientation of the matrix polymer. Opaque, microvoided filmshave been made in accordance with the methods of this invention usingcompletely amorphous, non-crystallizing copolyesters as the matrix,phase. Crystallizable/ orientable (strain hardening) matrix materialsare preferred for some properties like tensile strength and barrier. Onthe other hand, amorphous matrix materials have special utility in otherareas like tear resistance and heat sealability. The specific matrixcomposition can be tailored to meet many product needs. The completerange from crystalline to amorphous matrix polymer is part of theinvention.

Other ingredients are often added such as surfactants, emulsifiers,pigments, and the like during the preparation of such microbeads. Due tothe nature of these additives, they tend to remain on the surfaces ofthe microbeads. In other words, they tend to accumulate at the interfacebetween the polymer and the immiscible medium in which the suspensionpolymerization is carried out. However, due to the nature of suchprocesses, some of these materials can remain within the core of thebeads and some in the immiscible medium. For example, processing andformulating may be done to entrap ingredients within the beads. In othercases, the goal may be to concentrate ingredients on the surface of thebeads. It is this highly diverse and very controllable set of beadproperties that adds to the uniqueness of this invention.

Examples 1-36 which follow describe in considerable detail biaxiallyoriented films made with polyethylene terephthalate as the matrixpolymer. These examples are submitted for a better understanding of theinvention.

For the examples involving cross-linked microbeads, the preparationsteps are as follows:

(1) The microbeads are prepared by conventional aqueous suspensionpolymerization to give nearly monodisperse bead diameters from 2 to 20microns and at levels of cross-linking from 5 mol % to 30 mol %. Almostall of these examples employ coated microbeads, with the coatingthickness being about 50-100 nm.

(2) After separation and drying, the microbeads are compounded onconventional twin-screw extrusion equipment into the orientable polymerto a level of 25% by weight and pelletized to form a concentrate,suitable for let-down to lower loadings.

(3) The microbead concentrate pellets are mixed with virgin pellets anddried using standard conditions for polyethylene rerephthalate,170°-180° C. convection with desiccated air for 4-6 hours.

(4) The dried blends are extruded on conventional single-screw extrudersat melt temperatures at about 265°-280° C., standard conditions for thepolyethylene terephthalate used.

(5) Films are cast through a standard coat hanger slit die onto a chillroll controlled to a temperature about 50°-60° C. yielding films rangingfrom 20 to 60 mils (508-1524 microns) thick.

(6) The films are stretched biaxially and simultaneously using astandard laboratory film stretching unit. Unless otherwise specified allsamples are stretched at 105° C.

Examples involving cellulose acetate microbeads are included forcomparative purposes. The preparation procedure is as follows:

(1) The polyethylene terephthalate pellets are ground through a 2 mmscreen and dry-blended with the cellulose acetate powder.

(2) The blends are pan dried in a vacuum oven with dry nitrogen bleed atabout 125°-150° C. for 16 hours.

(3) The dried blends are simultaneously extruded and compounded onconventional single-screw extruders using a standard Maddock mixingsection in the metering region of the screw. Melt temperatures are keptas low as possible, about 260°-270° C., to minimize thermal degradationof the cellulose acetate.

(4) During the extrusion, molten CA microbeads form "in situ" by aprocess of shear emulsification and remain uniformly dispersed due totheir high immiscibility with the PET. A distribution of particlediameters is produced ranging from about 0.1-10 microns, with theaverage being about 1-2 microns.

(5) Films are cast through a standard coat hanger slit die onto a chillroll controlled to a temperature about 50°-60° C. yielding films rangingfrom 20 to 30 mils (508-761 microns) thick.

(6) The films are stretched biaxially and simultaneously using astandard laboratory film stretching unit. These films were stretched at100°, 105°, and 110° C.

The materials used in the examples are identified as follows:

PET - polyester having repeat units from terephthalic acid and ethyleneglycol; I.V.=0.70

CA - cellulose acetate, viscosity=3.0 seconds, 11.4 poises; acetylcontent 39.8%; hydroxyl content=3.5%; melting range=230°-250° C.;Tg=180° C.; number average molecular weight=30,000 (Gel PermeationChromatography)

PS - polystyrene cross-linked with divinylbenzene to various levels

PMMA - polymethylmethacrylate cross-linked with divinylbenzene tovarious levels

silica - colloidal silica, SiO₂, mean particle diameter=20-40 nmalumina - colloidal alumina, Al₂ O₃, mean particle diameter=20-40 nm

EXAMPLES 1-36

The examples illustrating this invention are organized into five groups,and the key information is summarized in a pair of tables for eachgroup. Examples 1 through 14 contain silica-coated, cross-linked PSmicrobeads (Tables 1 and 2). Here variations of microbead loading, levelof cross.-linking, and bead diameter are described along with the effectof stretch ratio (Examples 7-10). Example 1 is included to illustratethe invention when the coating has been intentionally removed from themicrobeads. The higher the cross-linking level, the more resilient orelastic the microbeads, and the efficiency of the cavitation process isincreased.

Examples 15 through 18 contain silica-coated, crosslinked PMMAmicrobeads (Tables 3 and 4). These samples illustrate the invention withthe same silica coating on microbeads of a different cross-linkedpolymer. The next two sets of samples repeat the demonstration with adifferent coating material. Examples 19 through 22 containalumina-coated, crosslinked PS microbeads (Tables 5 and 6); and Examples23 through 26 contain alumina-coated, cross-linked PMMA microbeads(Tables 7 and 8).

Finally, Examples 27 through 36 (Tables 9 and 10) compare, at equalvolume % loading, the performance of the silica-coated, 30%cross-linked, 5 micron PS microbeads of this invention to theperformance of in situ-generated, high Tg CA microbeads. Thesecomparisons clearly demonstrate a 20% increase in cavitating efficiencyfor this specific embodiment of the invention relative to the citedprior art.

                  TABLE 1                                                         ______________________________________                                        Biaxially Oriented Polyester (PET) Films Containing                           Silica-Coated, Cross-Linked Polystyrene (PS) Microbeads                              Mass                                                                   Example                                                                              Ratio    Cross-Link                                                                              Bead Diam.                                                                            Stretch                                                                             Specific                              Number PET/PS   Mole %    Microns Ratio Gravity                               ______________________________________                                         1     75/25    30        10      3.0   0.55                                   2     "         5        10      3.0   0.73                                   3     80/20    30        20      3.0   0.63                                   4     "         5        20      3.0   0.72                                   5     80/20    30        10      3.0   0.81                                   6     "         5        10      3.0   0.85                                   7     80/20    30        5       3.8   0.67                                   8     "        30        5       3.5   0.72                                   9     "        30        5       3.0   0.81                                  10     "        30        5       2.5   0.97                                  11 NC  "        30        5       3.0   0.92                                  12     "         5        5       3.0   1.06                                  13     "        30        2       3.0   1.00                                  14     "         5        2       3.0   1.03                                  ______________________________________                                         Note                                                                          "NC" indicates "no coating" on the microbeads                            

                  TABLE 2                                                         ______________________________________                                        Kubelka-Munk Analysis (@560 nm) of                                            Biaxially Oriented Polyester (PET) Films Containing                           Silica-Coated, Cross-Linked Polystyrene (PS) Microbeads                              Film             Scat-        Trans-                                          Thick-   Reflec- tering                                                                              Absorp-                                                                              mit-                                     Example                                                                              ness     tance   Coef. tance  tance Opac-                              Number Microns  R (inf) SX    KX     T (i) ity                                ______________________________________                                         1     297      91.1    6.38  0.028  12.5  0.931                               2     203      91.6    5.83  0.023  13.8  0.917                               3     295      92.3    3.59  0.012  20.4  0.849                               4     229      93.3    4.99  0.012  16.2  0.885                               5     203      91.1    3.58  0.015  21.2  0.848                               6     224      91.0    5.75  0.026  13.8  0.920                               7     127      95.2    5.20  0.006  15.9  0.876                               8     137      95.0    5.05  0.006  16.4  0.873                               9     183      93.0    6.36  0.016  13.1  0.917                              10     206      90.6    4.67  0.023  16.8  0.894                              11 NC  150      97.1    3.21  0.001  23.7  0.785                              12     132      97.0    1.00  0.001  50.0  0.515                              13     155      93.8    6.80  0.014  12.3  0.920                              14     137      92.5    4.04  0.012  19.3  0.859                              ______________________________________                                         Note                                                                          "NC" indicated "no coating" on the microbeads                            

                  TABLE 3                                                         ______________________________________                                        Biaxially Oriented Polyester (PET) Films                                      Containing Silica-Coated, Cross-Linked                                        Polymethylmethacrylate (PMMA) Microbeads                                             Mass                                                                   Example                                                                              Ratio    Cross-Link                                                                              Bead Diam.                                                                            Stretch                                                                             Specific                              Number PET/PS   Mole %    Microns Ratio Gravity                               ______________________________________                                        15     80/20    30        5       3.0   0.90                                  16     "         5        5       3.0   1.06                                  17     80/20    30        2       3.0   1.09                                  18     "         5        2       3.0   1.23                                  ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Kubelka-Munk Analysis (@560 nm) of                                            Biaxially Oriented Polyester (PET) Films                                      Containing Silica-Coated, Cross-Linked                                        Polymethylmethacrylate (PMMA) Microbeads                                             Film             Scat-        Trans-                                          Thick-   Reflec- tering                                                                              Absorp-                                                                              mit-                                     Example                                                                              ness     tance   Coef. tance  tance Opac-                              Number Microns  R (inf) SX    KX     T (i) ity                                ______________________________________                                        15     163      94.9    2.88  0.004  25.6  0.780                              16     142      96.8    2.23  0.001  30.9  0.712                              17     117      92.9    3.68  0.010  20.9  0.840                              18     122      92.6    1.66  0.005  37.4  0.671                              ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        Biaxially Oriented Polyester (PET) Films                                      Containing Alumina-Coated, Cross-Linked                                       Polystyrene (PS) Microbeads                                                          Mass                                                                   Example                                                                              Ratio    Cross-Link                                                                              Bead Diam                                                                             Stretch                                                                             Specific                              Number PET/PS   Mole %    Microns Ratio Gravity                               ______________________________________                                        19     80/20    30        5       3.0   0.77                                  20     "         5        5       3.0   0.89                                  21     80/20    30        2       3.0   0.96                                  22     "         5        2       3.0   0.96                                  ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        Kubelka-Munk Analysis (@560 nm) of                                            Biaxially Oriented Polyester (PET) Films                                      Containing Alumina-Coated, Cross-Linked                                       Polystyrene (PS) Microbeads                                                          Film             Scat-        Trans-                                          Thick-   Reflec- tering                                                                              Absorp-                                                                              mit-                                     Example                                                                              ness     tance   Coef. tance  tance Opac-                              Number Microns  R (inf) SX    KX     T (i) ity                                ______________________________________                                        19     185      86.0    4.74  0.054  15.4  0.926                              20     150      86.5    4.55  0048   16.2  0.916                              21     155      91.6    6.30  0.024  12.8  0.926                              22     152      94.0    5.05  0.010  16.2  0.882                              ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        Biaxially Oriented Polyester (PET) Films                                      Containing Alumina-Coated, Cross-Linked                                       Polymethylmethacrylate (PMMA) Microbeads                                             Mass                                                                   Example                                                                              Ratio    Cross-Link                                                                              Bead Diam.                                                                            Stretch                                                                             Specific                              Number PET/PS   Mole %    Microns Ratio Gravity                               ______________________________________                                        23     80/20    30        5       3.0   0.90                                  24     "         5        5       3.0   1.08                                  25     80/20    30        2       3.0   0.88                                  26     "         5        2       3.0   1.11                                  ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                        Kubelka-Munk Analysis (@560 nm) of                                            Biaxially Oriented Polyester (PET) Films                                      Containing Alumina-Coated, Cross-Linked                                       Polymethylmethacrylate (PMMA) Microbeads                                             Film             Scat-        Trans-                                          Thick-   Reflec- tering                                                                              Absorp-                                                                              mit-                                     Example                                                                              ness     tance   Coef. tance  tance Opac-                              Number Microns  R (inf) SX    KX     T (i) ity                                ______________________________________                                        23     168      89.1    3.28  0.022  22.4  0.847                              24     135      90.8    1.80  0.008  35.3  0.703                              25     130      89.8    4.76  0.028  16.3  0.903                              26     160      76.8    1.60  0.056  35.8  0.766                              ______________________________________                                    

                  TABLE 9                                                         ______________________________________                                        Biaxially Oriented Polyester (PET) Films                                      Containing Silica-Coated, 30% Cross-Linked Polystyrene (PS),                  5 Micron Microbeads Versus Those                                              Containing Cellulose Acetate (CA) Microbeads                                  Example Bead*    Bead   Stretch Stretch                                                                              Specific                               Number  Wt. %    Type   Temp. °C.                                                                      Ratio  Gravity                                ______________________________________                                        27      20       CA     100     3.5    0.90                                   28      16       PS     100     3.5    0.80                                   29      20       CA     100     3.0    1.02                                   30      16       PS     100     3.0    0.88                                   31      20       CA     105     3.0    0.92                                   32      16       PS     105     3.0    1.04                                   33      20       CA     110     3.0    0.96                                   34      16       PS     110     3.0    1.12                                   35      20       CA     110     3.5    0.92                                   36      16       PS     110     3.5    0.94                                   ______________________________________                                         *Note                                                                         Volume % of the microbeads is equal since the specific gravities of PS an     CA are 1.05 and 1.32 respectively                                        

                  TABLE 10                                                        ______________________________________                                        Kubelka-Munk Analysis (@560 nm) of                                            Biaxially Oriented Polyester (PET) Films                                      Containing Silica-Coated, 30% Cross-Linked Polystyrene (PS),                  5 Micron Microbeads Versus Those                                              Containing Cellulose Acetate (CA) Microbeads                                         Film             Scat-        Trans-                                          Thick-   Reflec- tering                                                                              Absorp-                                                                              mit-                                     Example                                                                              ness     tance   Coef. tance  tance Opac-                              Number Microns  R (inf) SX    KX     T (i) ity                                ______________________________________                                        27     71       96.8    7.73  0.004  11.3  0.912                              28     79       97.4    6.72  0.002  12.9  0.892                              29     88       96.5    6.80  0.004  12.7  0.900                              30     96       96.7    6.31  0.004  13.6  0.890                              31     94       97.8    5.71  0.001  14.9  0.869                              32     84       97.5    5.94  0.002  14.3  0.877                              33     89       98.6    5.06  0.001  16.5  0.846                              34     76       97.2    5.00  0.002  16.6  0.856                              35     69       98.1    5.64  0.001  15.0  0.865                              36     74       97.4    5.54  0.002  15.2  0.869                              ______________________________________                                    

EXAMPLE 37

Sheet material comprising a polypropylene matrix having microbeads ofcross-linked polystyrene is prepared by mixing the microbeads in moltenpolypropylene and extruding a sheet. The sheet is oriented by stretchingat 140° C. in both directions. The microbeads are 5 microns in diameterand account for 15% by weight of the polypropylene. The microbeads arecross-linked with divinylbenzene and are cross-linked withdivinylbenzene and are cross-linked to different degrees, indicated withspecific gravity as follows:

    ______________________________________                                        % Cross-Linked Specific Gravity                                               ______________________________________                                        25             0.36                                                           20             0.43                                                           15             0.41                                                           10             0.48                                                           ______________________________________                                    

A very white sheet having good hand is obtained.

The following examples illustrate the invention with respect to twoimportant orientable polyesters. Due to the higher orientationtemperatures involved with these matrix polyesters, microbeads with thehigher level of cross-linking (about 40 mol %, i.e., 60 mol %polystyrene and 40 mol % divinylbenzene): are used.

EXAMPLE 38

A blend is prepared comprising 85 parts by weightpoly(1,4-cyclohexylenedimethylene terephthalate) and 15 partscross-linked polystyrene. The polyester (I.V.=0.61) is ground through a2 mm screen, dry blended with the microbeads, vacuum-oven dried, andcompounded at 290° C. on a laboratory-sized co-rotating, twin-screwextruder. The pellets are then dried and extruded on a single-screwextruder to make 5.5 inch wide film 20 mils thick. The films were thenbiaxially oriented (3×3) at 115° C. The resulting films showed the highdegree of whiteness and opacity as well as desirable low densitydiscussed in greater detail in the earlier examples.

EXAMPLE 39

In the same manner as the previous example, a blend is preparedcomprising 85 parts by weight poly(ethylene naphthalate) and 15 partscross-linked polystyrene. The polyester (I.V.=0.71) is ground through a2 mm screen, dry blended with the microbeads, vacuum-oven dried, andcompounded at 290° C. on a co-rotating, twin-screw extruder. The pelletsare then dried and extruded on a single-screw extruder to make 5.5 inchwide film 20 mils thick. The films are then biaxially oriented (3×3) at145° C. The resulting films showed the high degree of whiteness andopacity as well as desirable low density discussed in greater detail inthe carlier examples.

Where ratios or parts are given, e.g., 80/20, they are parts by weight,with the polyester weight specified first.

The following applies to Kubelka-Munk values:

SX is the scattering coefficient of the whole thickness of the articleand is determined as follows: ##EQU1## wherein:

    b=(a.sup.2 -1)1/2

Ar ctgh is the inverse hyperbolic cotangent ##EQU2## Ro is reflectancewith black tile behind sheet R is reflectance with white tile behindsheet

Rg is reflectance of a white tile=0.89

KX is the absorption coefficient of the whole thickness of the articleand is determined as follows:

    KX=SX(a-1)

wherein SX and a are as defined above

R (infinity) is the reflectance of an article if the article was sothick that additional thickness would not change it and is determined asfollows:

    R(infinity)=a-(a.sup.2 -1)1/2

wherein a is as defined above

Ti is the internal light transmittance and is determined as follows:##EQU3## wherein Ro and Rg are as defined above.

In the above formulae, Ro, R and Rg are determined in a conventionalmanner using a Diano Match-Scan II Spectrophotometer (Milton Roy Co.)using a wavelength of 560 nanometers. Also above, X in the formulae SXand KX is the thickness of the article. A full description of theseterms is found in Colors in "Business, Science and Industry" 3rdEdition, by Deane B. Judd & Gunter Wyszecki, published by John Wiley &Sons, N.Y. (1975), pages 397-439, which is incorporated herein byreference.

Glass transition temperatures, Tg and melt temperatures, Tm, aredetermined using a Perkin-Elmer DSC-2 Differential Scanning Calorimeter.

Unless otherwise specified inherent viscosity is measured in a 60/40parts by weight solution of phenol/tetrachloroethane 25° C. and at aconcentration of about 0.5 gram of polymer in 100 ml of the solvent.

Where acids are specified herein in the formation of the polyesters orcopolyesters, it should be understood that ester forming derivatives ofthe acids may be used rather than the acids themselves as isconventional practice. For example, dimethyl isophthalate may be usedrather than isophthalic acid.

Unless otherwise specified, all parts, ratios, percentages, etc. are byweight.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

We claim:
 1. A shaped article selected from the group .[.comprising.]..Iadd.consisting of .Iaddend.films, sheets, bottles, tubes, fibers androds wherein said article comprises a continuous oriented polymer phaseselected from the group .[.comprising.]. .Iadd.consisting of eppolyesters and polypropylene having dispersed therein microbeads of across-linked polymer coated with a slop agent and which are at leastpartially bordered by void space .Iadd.formed from substantially closedcells so that substantially no open fluid transport path extends acrossopposed sides of said article.Iaddend., said microbeads being present inan amount of about 5-50% by weight based on the weight of said orientedpolymer, said void space occupying about 2-60% by volume of said shapedarticle wherein said cross-linked polymer comprises polymerizableorganic material which is a member selected from the group consisting ofan alkenyl aromatic compound having the general formula ##STR4## whereinAr represents an aromatic hydrocarbon radical, or an aromatic.[.halohydracarbon.]. .Iadd.halohydrocarbon .Iaddend.radical of thebenzene series and R is hydrogen or the methyl radical; acrylate-typemonomers include monomers of the formula ##STR5## wherein R is selectedfrom the group consisting of hydrogen and an alkyl radical containingfrom about 1 to 12 carbon atoms and R' is selected from the groupconsisting of hydrogen and methyl; copolymers of vinyl chloride andvinylidene chloride, acrylonitrile and vinyl chloride, vinyl bromide,vinyl esters having the formula ##STR6## wherein R is an alkyl radicalcontaining from 2 to 18 carbon atoms; acrylic acid, methacrylic acid,itaconic acid, citraconic acid, maleic acid, fumaric acid, oleic acid,vinylbenzoic acid; the synthetic polyester resins which are prepared byreacting terephthalic acid and dialkyl terephthalics or ester-formingderivatives thereof, with a glycol of the series HO(CH₂)_(n) OH, whereinn is a whole number within the range of 2-10 and having reactiveolefinic linkages within the polymer molecule, the hereinabove describedpolyesters which include copolymerized therein up to 20 percent byweight of a second acid or ester thereof having reactive olefinicunsaturation and mixtures thereof, and a cross-linking agent selectedfrom the group consisting of divinylbenzene, diethylene glycoldimethacrylate, oiallyl fumarate, diallyl phthalate and mixturesthereof.
 2. A shaped article according to claim 1, .[.the composition ofwhich when consisting only of said continuous phase and said microbeadsat least partially bordered by void space is characterized by.]..Iadd.having a composition, which when consisting only of saidcontinuous phase and said microbeads at least partially bordered by voidspace, is characterized by .Iaddend.having a specific gravity of lessthan 1.20 and a Kubelka-Munk R value (infinite thickness) of about 0.90to about 1.0 and the following Kubelka-Munk values when formed into a 3mil thick film:

    ______________________________________                                        Opacity          about 0.78 to about 1.0                                      SX               25 or less                                                   KX               about 0.001 to 0.2                                           T (i)            about 0.02 to 1.0                                            ______________________________________                                    


3. A shaped article according to claim 1 wherein said oriented polymerphase is poly(ethylene terephthalate) having an I.V. of at least 0.50.4. A shaped article according to claim 1 wherein said oriented polymerphase is a polyester having repeat units from terephthalic acid ornaphthalene dicarboxylic acid and at least one glycol selected from thegroup consisting of ethylene glycol, 1,4-butanediol and1,4-cyclohexanedimethanol.
 5. A shaped article according to claim 1wherein said oriented polymer phase is polypropylene.
 6. A shapedarticle according to claim 1 wherein said slip agent is selected fromsilica and alumina.
 7. A shaped article according to claim 1 whereinsaid microbeads have an average diameter of about 0.1-50 microns.
 8. Ashaped article according to claim 1 wherein said void spaces surroundsaid microbeads, said void spaces being oriented such that they lie ingenerally the same or parallel planes.
 9. A shaped article according toclaims 1, 2, 3, 4,.5, 6, 7 or 8 wherein said article is a fiber or rodof about 0.5-50 mils diameter.
 10. A shaped article according to claims1, 2, 3, 4, 5, 6, 7, or 8 wherein said article is a tube.
 11. A shapedarticle according to claims 1, 2, 3, 4, 5, 7 or 8 wherein said articleis a paper-like sheet.
 12. A shaped article according to claim 1, 2, 3,4, 5, 7 or 8 wherein said article is a bottle.
 13. A paper-like sheetcomprising a continuous phase of oriented polyester having dispersedtherein micro-beads of crosslinked polymer coated with a slip agentwhich are encircled by void space .Iadd.formed from substantially closedcells so that substantially no open fluid transport path extends acrossopposed sides of said paper-like sheet .Iaddend.when viewed in adirection perpendicular to the plane of the sheet,(a) said polyesterhaving a Tg of about 60°-100° C. and an I.V. of at least 0.50, (b) saidcrosslinked polymer comprises polymerizable organic material which is amember selected from the group consisting of an alkenyl aromaticcompound having the general formula ##STR7## wherein Ar represents anaromatic hydrocarbon radical, or an aromatic .[.halohydracarbon.]..Iadd.halohydrocarbon .Iaddend.radical of the benzene series and R ishydrogen or the methyl radical; acrylate-type monomers include monomersof the formula ##STR8## wherein R is selected from the group consistingof hydrogen and an alkyl radical containing from about 1 to 12 carbonatoms and R' is selected from the group consisting of hydrogen andmethyl; copolymers of vinyl chloride and vinylidene chloride,acrylonitrile and vinyl chloride, vinyl bromide, vinyl esters having theformula ##STR9## wherein R is an alkyl radical containing from 2 to 18carbon atoms; acrylic acid, methacrylic acid, itaconic acid, citraconicacid, maleic acid, fumaric acid, oleic acid, vinylbenzoic acid; thesynthetic polyester resins which are prepared by reacting terephthalicacid and dialkyl terephthalics or ester-forming derivatives thereof,with a glycol of the series HO(CH₂)_(n) OH, wherein n is a whole numberwithin the range of 2-10 and having reactive olefinic linkages withinthe polymer molecule, the hereinabove described polyesters which includecopolymerized therein up to 20 percent by weight of a second acid orester thereof having reactive olefinic unsaturation and mixturesthereof, and a cross-linking agent selected from the group consisting ofdivinylbenzene, diethylene glycol dimethacrylate, oiallyl fumarate,diallyl phthalate and mixtures thereof, (c) said microbeads having anaverage diameter of about 0.1-50 microns and being present in an amountof about 5-50% by weight based on the weight of said polyester, and (d)said void space occupying about 2-60% by volume of said sheet.
 14. Apaper-like sheet according to claim 13 wherein said slip agent isselected from silica and alumina.
 15. A fiber or rod comprising acontinuous phase of oriented polyester having dispersed thereinmicrobeads comprising crosslinked polymer coated with a slipagent.Iadd., wherein said microbeads are .Iaddend.bounded on thelengthwise sides by void space .Iadd.formed from substantially closedcells so that substantially no open fluid transport path extends acrossopposed sides of said fiber or said rod(a) said oriented polyesterhaving a Tg of about 60°-100° C. and an I.V. of at least 0.50, (b) saidmicrobeads having an average diameter of about 0.1-50 microns andaccounting for 10-30% by weight of said .[.sheet.]. .Iadd.fiber or saidrod.Iaddend.ol , and (c) said void space occupying about 2-60% by volumeof said fiber or rod.
 16. A fiber or rod according to claim 15 whereinsaid crosslinked polymer comprises polymerizable organic material whichis a member selected from the group consisting of an alkenyl aromaticcompound having the general formula ##STR10## wherein Ar represents anaromatic hydrocarbon radical, or an aromatic halohydrocarbon radical ofthe benzene series and R is hydrogen or the methyl radical;acrylate-type monomers include monomers of the formula ##STR11## whereinR is selected from the group consisting of hydrogen and an alkyl radicalcontaining from about 1 to 12 carbon atoms and R' is selected from thegroup consisting of hydrogen and methyl; copolymers of vinyl chlorideand vinylidene chloride, acrylonitrile and vinyl chloride, vinylbromide, vinyl esters having the formula ##STR12## wherein R is an alkylradical containing from 2 to 18 carbon atoms; acrylic acid, methacrylicacid, itaconic acid, citraconic acid, maleic acid, fumaric acid, oleicacid, vinylbenzoic acid; the synthetic polyester resins which areprepared by reacting terephthalic acid and dialkyl terephthalics orester-forming derivatives thereof, with a glycol of the seriesHO(CH₂)_(n) OH, wherein n is a whole number with the range of 2-10 andhaving reactive olefinic linkages within the polymer molecule, thehereinabove described polyesters which include copolymerized therein upto 20 percent by weight of a second acid or ester thereof havingreactive olefinic unsaturation and mixtures thereof, and a crosslinkingagent selected from the group consisting of divinylbenzene, diethyleneglycol dimethacrylate, oiallyl fumarate, diallyl phthalate and mixturesthereof.
 17. A fiber or rod according to claim 16 wherein said slipagent is selected from silica and alumina.